We recently completed the first comprehensive study of tyrosine phosphorylated substrates in resting and BCR-stimulated primary B cells. From this effort, we focused on a novel transmembrane adaptor protein, termed Twixt, due to its novelty, selective expression in B cells and potential importance in fulfilling a knowledge gap in our understanding of BCR signaling - namely how BLNK is inducibly recruited to the BCR complex. In the proposed work, we will determine how Twixt is recruited and utilized by the BCR and perhaps other receptors to effect downstream signaling. These data will provide a mechanistic framework for interpreting the phenotypes observed in Twixt-deficient mice, which will be generated and analyzed in the second Aim. Completion of the proposed studies will establish the prominence of Twixt as a central player in BCR signaling, or reveal a more selective role.